1. Field of the Invention
The invention relates to a thermal stenciling device having a thermal head constructed of a plurality of heat generating elements for perforating a heat-sensitive stencil paper.
2. Description of Related Art
In a conventional thermal stenciling device, it is known that a thermal head constructed of a plurality of heat generating elements is pressed against a thermoplastic resin film of a heat-sensitive stencil paper to thermally perforate the thermoplastic resin film.
FIG. 27 is a schematic illustration of such a thermal stenciling device in the prior art. Referring to FIG. 27, a heat-sensitive stencil paper 1, held between a pair of driven rollers 2, is fed by a platen roller 3 in a direction depicted by an arrow A, and is pressed by a plurality of heat generating elements 5 of a thermal head 40 against the platen roller 3. More specifically, a thermoplastic resin film formed on an upper side 1a of the heat-sensitive stencil paper 1 is pressed by the heat generating elements 5 of the thermal head 40, and the thermoplastic resin film of the stencil paper 1 is thermally perforated by the heat generating elements 5. In general, a thermal head such as employed in a facsimile apparatus is used as the thermal head 40.
FIG. 28 is a sectional view of the stencil paper 1. Referring to FIG. 28, the stencil paper 1 is constructed of a thermoplastic resin film 12, an adhesive layer 13 and a porous carrier 14. The thermoplastic resin film 12 and the porous carrier 14 are bonded together by the adhesive layer 13. The thermoplastic resin film 12 is formed as a polyethylene terephthalate (which will be hereinafter abbreviated as a PET film) having a thickness of 2 .mu.m. Other materials may be used for the thermoplastic resin film 12, such as polypropylene and vinylidene chloride-vinyl chloride copolymer. The porous carrier 14 is formed as a porous thin sheet primarily composed of a natural fiber, such as Manila hemp, kozo or mitzumata; a synthetic fiber such as polyethylene terephthalate, polyvinyl alcohol or polyacrylonitrile; or a semisynthetic fiber such as rayon.
FIG. 3A is a schematic plan view of the thermal head 40. In FIG. 3A, a direction of feed of the stencil paper 1, that is, a direction of relative movement of the thermal head 40 is defined as a sub-scanning direction, and a direction perpendicular to the sub-scanning direction is defined as a main-scanning direction. The heat generating elements 5 each having a rectangular shape are arranged in line in the main-scanning direction of the thermal head 40. Two pattern layers 6 are connected to the opposite ends of each heat generating element 5 in the sub-scanning direction, so as to supply an electric power to each heat generating element 5.
When the electric power is supplied to the heat generating elements 5 of the thermal head 40, they generate heat. Accordingly, the temperature of the thermoplastic resin film 12 of the stencil paper 1 contacting the heat generating elements 5 under pressure is increased by the heat generated from the heat generating elements 5. When the temperature of the thermoplastic resin film 12 becomes higher than a shrinkage starting temperature Ta, the film 12 is melted to initially generate fine perforations and then enlarge them. Thereafter, when the supply of the electric power to the heat generating elements 5 is stopped, the heat of the heat generating elements 5 is radiated. Accordingly, the temperature of the thermoplastic resin film 12 is decreased to become lower than a shrinkage ending temperature Tb. As a result, the growth of the perforations formed through the thermoplastic resin film 12 is terminated, and the perforations are fixed.
In a facsimile apparatus, a feed rate of a recording paper in the sub-scanning direction is pre-established. Accordingly, the size of each heat generating element 5 of the thermal head 40 is determined based upon the pre-established feed rate.
In the conventional thermal head 40 for a facsimile, a ratio between a length b of each heat generating element 5 in the sub-scanning direction and a dot pitch Pb of the heat generating elements 5 in the sub-scanning direction is set to about 2:1, that is, the ratio b:Pb of approximately 2:1 is set to ensure constant print in the sub-scanning directing without white lines between adjacent dots that are meant to be connected. Accordingly, as shown in FIG. 3B, heat generating regions of the perforation dots to be formed on the stencil paper 1 overlap each other at D in the sub-scanning direction at given intervals.
However, in the conventional thermal stenciling device as mentioned above, there is possibility that a surface temperature of the thermoplastic resin film 12 at a gap portion between the adjacent dots in the main-scanning direction becomes higher than the shrinkage ending temperature Tb as a result of the thermal energy applied from the heat generating elements 5 to the stencil paper 1. In this case, the perforation generated at the center of each dot grows, and does not terminate in the gap portion but reaches the adjacent dot, thus forming a continuous perforation in the main-scanning direction. Further, since the overlap portion D exists between the adjacent dots in the sub-scanning direction, the above continuous perforation becomes continuous also in the sub-scanning direction.
As a result, in forming a solid image, a large, continuous perforation in both the main- and sub-scanning directions, with no gap portions, is formed on the stencil paper 1. Accordingly, the melted resin of the film 12, in a fluidic condition, becomes entangled with the porous carrier 14 to again form a thickened resin film or bulk. The resin film or bulk thus formed causes the formation of an undue white image portion in a black image portion in printing resulting in the appearance of so-called "Japanese paper crimp" in a printed image.
Further, a quantity of ink to be transferred through the large perforation onto a printing paper is larger than that through other image portions. Accordingly, the phenomena of undrying, bleeding and back imaging on the printing paper are frequent in the solid image. Further, character images and line images are also formed by perforation dots continuous in both the main-and sub-scanning directions, so that the phenomena of undrying, bleeding and back imaging become frequent also in character images and the line images.